1. Field of the Invention
The present invention is related to a cruise control system for a motor vehicle and more particularly to a cruise control system having an automatic restoration function when brakes are applied.
2. Discussion of the Prior Arts
In a tendency of increasing number of traffic accidents, it has been no longer adequate merely to enhance the safety of vehicles themselves. One aspect for decreasing car accidents is to prevent vehicles from falling into hazardous situations, that is, a so-called ADA system (Active Drive Assist System) for positively assisting drivers' safe driving has been developed in universities, institutes, automobile manufacturers and the like.
One example of such ADA system includes a monitoring camera mounted on a vehicle for recognizing road situations, obstacles, traffic environments and other information and based on these information the system raises alarm for urging a driver to apply operations to the vehicle so as to avoid hazard. If the driver does not notice the alarm or neglects it, temporary drive assisting means are taken in such away that brake is automatically applied or a steering wheel is operated automatically on the safe side.
Further, the ADA system, since it can detect a vehicle speed of a preceding vehicle, an inter-vehicle distance, a relative vehicle speed and the like, has a capability of following behind the preceding (tentatively, referred to as a following-behind control) while a safe inter-vehicle distance is retained between the subject vehicle and the preceding vehicle. The ADA system combined with a cruise control system can perform a constant speed control when the preceding vehicle does not exist or exists far away from the vehicle. When a preceding vehicle travels nearby, the cruise control system is switched to the following-behind control as described above so as to follow the preceding vehicle from behind.
Generally, the cruise control system is released automatically when applying brakes, in order to prevent the throttle opening angle from increasing inversely during deceleration by braking operation. Therefore, in case where a driver desires to restore the cruise control after braking operation, he must switch the cruise control ON again. When this automatic release of the cruise control is applied to the following-behind control, there occurs a following problem.
When the subject vehicle catches up with the preceding vehicle and the vehicle transfers into the following-behind control, the cruise control system operates to reduce the vehicle speed by decreasing the throttle opening angle. In this case, in order to hasten the transference into the following-behind control, when the driver applies brake during this transfer process to the following-behind control, not only the cruise control but also the following-behind control is released. As a result, the driver must switch the cruise control ON again and this provides the driver with much inconvenience.
Further, after the cruise control changes into the following-behind control, when the preceding vehicle goes out of the monitoring range, the vehicle returns to the cruise control. At this moment, if the preset or target vehicle speed stays in a relatively high value, the vehicle starts a steep acceleration so as to try to restore the preset vehicle speed. This type of the cruise control system is undesirable in terms of drive feeling and should be improved in view of safety.
With respect to the following-behind control as described above, Japanese Patent Application Laid-open No. Toku-Kai-Hei 4-260836 discloses a technique in which the following-behind control is performed automatically according to the inter-vehicle distance between the self vehicle and the preceding vehicle and, when the variation of vehicle speeds of the self vehicle is small, the following-behind control is changed over into the constant speed control, whereby drive feeling can be prevented from being exacerbated.
However, this prior art has a problem in that once a brake is applied during the constant speed control, the cruise control system is canceled, and also a problem of the above mentioned steep acceleration when restoring the preset vehicle speed.